The invention relates to a static induction or field-effect bipolar transistor with voltage control, suitable for high-voltage or high-current operations.
A conventional bipolar transistor has a three-layer structure such as NPN or a PNP structure in which an emitter-collector current is controlled by a base current. The current-amplification factor, a measure for the transistor's control performance, is determined by the base width, i.e., the thickness of the base layer along the current-flow direction. The current-amplification factor can be increased by narrowing the base width, thereby enhancing the control performance. By incorporating a base having substantially zero width, and by using a static induction or magnetic field effect, control performance is greatly improved over the conventional bipolar transistor; see IEDM Technical Digest, 1978, p. 676; Japanese Journal of Applied Physics 17 (1978), p. 245; and IEEE Electron Device Letters EDL-6 (1985), p. 522. These articles variously describe transistor structures designated as BSIT, DBT and EMFET. As used in the following, the designation BSIT stands for bipolar static induction transistor.
Basic structure and operation of a conventional BSIT are briefly explained hereunder with reference to FIG. 4. A semiconductor body 10 for the BSIT 30 consists of an n-type epitaxial layer 12 several tens of micrometers thick as a collector region on an n.sup.+ -type substrate 11, for example. Two p-type base regions 13 are formed by diffusion at the surface of the collector region 12, with the thickness of the base regions 13 being greater than in earlier conventional structures, as shown in FIG. 4. An n.sup.+ -type emitter region 14 is diffused at the surface of the collector region 12, between the two base regions 13. A collector terminal C, an emitter terminal E, and a base terminal B are respectively connected, via electrode films 25, to the substrate 11, the emitter region 14, and the base regions 13.
For the sake of exposition, FIG. 4 shows a hypothetical base region 13a below the emitter region 14. If a base region 13a were actually present, the structure of a vertical NPN transistor would be realized. Substantially lacking a base region 13a, the BSIT 30 has minimized base width (which is the thickness of the base region 13a), and a static induction effect is utilized by which the collector region 12 is influenced by the two surrounding base regions 13.
When the BSIT 30 is turned off, a depletion region extends from a p-n junction between the collector region 12 and the base regions 13, mainly towards the collector region 12, because of a positive voltage applied to the collector terminal C. When a base current flows into the base regions 13 from the base terminal B, a hole current h flows into the emitter region 14 from the base region 13 through the p-n junction in the forward direction. An electron current e is generated from the emitter region 14 in correspondence with this current flow. Part of this current e flows into the base region 13 through the p-n junction, but most of the electrons flow into the collector region 12 because of the static induction or electric field effect of the collector region 12. As a result, there is conduction between the collector and the emitter. To switch from this on-state to the off-state, a reverse bias is applied between the base region 13 and the collector region 12 via the base terminal B, thereby expanding the depletion region, with the current in the collector region 12 from the p-n junction generated from both sides of the p-n junction.
In a BSIT 30 with the aforementioned basic structure and operation, because the width of the hypothetical base region 13a is infinitesimally small, a high current-amplification factor can be realized. The vertical construction allows increased thickness of the collector region 12 and extension of the depletion at turn-off, thereby allowing the transistor to withstand high voltages. If this BSIT 30 is compared with a field-effect transistor, the collector, emitter and the base correspond to drain, source and gate, respectively.
As described, a BSIT has an advantageously high current-amplification factor. However, in a BSIT, the current between the collector and the emitter, or the on-off states, are controlled by the base current. Thus, from the point of view of circuit control, a BSIT more closely resembles a bipolar transistor than a field-effect transistor. The low output impedance of the BSIT is an advantage, but the low input impedance is a disadvantage. Furthermore, to increase the current-carrying capacity, a plurality of unit structures (per FIG. 4) are connected in parallel on a common substrate, for example. In this case, as the current tends to concentrate on particular unit structures, such structures may be damaged due to overcurrent.